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The backup copy now fsyncs its temp file before the rename and the directory after it, using the download writer's new fsyncPath helper. Each backup run deletes .quak-backup-*.tmp files whose process is no longer running, leaving those of a concurrent backup alone. The rename sites and the README backup layout state that a symlink at the destination is replaced and the new file takes the temp file's permissions, and the README names the temp files. Adds tests for a missing and an unwritable destination directory for downloadFile and downloadThumbnail. Model: opus-5-5